Analytical Characterization and Optimum Performance of Nonlinearly Distorted Coherent Optical OFDM Signals

Abstract

Nonlinear phase noise can severely degrade the performance of multicarrier optical signals by introducing both inband and out-of-band distortion. In this paper we study analytically the impact of nonlinear phase distortion on CO-OFDM (Coherent Optical Orthogonal Frequency Division Multiplexing) schemes with large number of subcarriers. By using the Bussgang theorem, we derive the power spectral densities of the useful and the distortion term of the nonlinearly distorted signal. The traditional approach in conventional CO-OFDM implementations is to treat nonlinear distortion as an undesirable noise term that leads to performance degradation. However, it can be shown that this distortion has information on the transmitted signals that can be used to improve the performance. Surprisingly, we show that the distortion inherent to the nonlinear phase noise can lead to performance improvements relatively to the ideal, linear CO-OFDM transmissions. We present a closed-form expression for the average asymptotic gain associated to the optimum detection of nonlinearly distorted CO-OFDM signals and we derive the approximated bit-error-rate using the distribution of the the asymptotic gain.